Problem 211 - Chemistry Matter and Change

Question

Write balanced equations for each reaction. (Chapter 9) a. Magnesium metal and water combine to form solid magnesium hydroxide and hydrogen gas. b. Dinitrogen tetroxide gas decomposes into nitrogen dioxide gas. c. Aqueous solutions of sulfuric acid and potassium hydroxide undergo a double-replacement reaction.

Step-by-Step Solution

Answer

a. \(Mg + 2 H_2O \rightarrow Mg(OH)_2 + H_2\) b. \(N_2O_4 \rightarrow 2 NO_2\) c. \(H_2SO_4 + 2 KOH \rightarrow K_2SO_4 + 2 H_2O\)

Step 1: a. Magnesium metal and water combine to form solid magnesium hydroxide and hydrogen gas.

First, write down the reactants and products for the reaction: unbalanced equation is: Mg + H2O -> Mg(OH)2 + H2 Now, balance the equation by adjusting the coefficients: Mg + 2 H2O -> Mg(OH)2 + H2 This is the balanced equation for the reaction between magnesium metal and water.

Step 2: b. Dinitrogen tetroxide gas decomposes into nitrogen dioxide gas.

First, we write down the reactants and products for the reaction: unbalanced equation is: N2O4 -> NO2 Now, balance the equation: N2O4 -> 2 NO2 This is the balanced equation for the decomposition of dinitrogen tetroxide gas.

Step 3: c. Aqueous solutions of sulfuric acid and potassium hydroxide undergo a double-replacement reaction.

First, write down the reactants and products for the reaction: unbalanced equation is: H2SO4 + KOH -> K2SO4 + H2O Now, balance the equation: H2SO4 + 2 KOH -> K2SO4 + 2 H2O This is the balanced equation for the double-replacement reaction between aqueous solutions of sulfuric acid and potassium hydroxide.

Key Concepts

Chemical Reactions

A chemical reaction is a process where reactants are transformed into products through the breaking and forming of chemical bonds. When magnesium metal reacts with water, the magnesium atoms lose electrons to become magnesium ions, while water molecules gain those electrons, leading to the formation of magnesium hydroxide and hydrogen gas. This transformation is depicted in a chemical equation that must be balanced to reflect the law of conservation of mass. To enhance comprehension, thinking of chemical reactions as simple exchanges or transformations where no atoms are lost or gained can be helpful—it's like a dance where partners (atoms) may switch, but everyone is still on the dance floor.

For students struggling to grasp chemical reactions, a useful tip would be to visualize the process. Imagine starting with a certain number of each type of atom (such as magnesium, hydrogen, and oxygen), and ensuring that the same number of those atoms appear on the product side. It's like having a set of building blocks and rearranging them to build something new without gaining or losing any blocks.

Stoichiometry

Stoichiometry is the quantitative aspect of chemistry that relates the amount of reactants to products in a chemical reaction. It involves using balanced equations to calculate the exact proportions in which substances react and form. In the case of magnesium and water, the stoichiometric calculation ensures that there are two water molecules for each magnesium atom, to fully react and form one magnesium hydroxide molecule and one hydrogen molecule. To better understand stoichiometry, students might think of it as a recipe that outlines the exact amount of ingredients needed to make a dish—with too much or too little of something, the dish won't come out right.

To aid students, breaking down the concept into more manageable parts such as: identifying the 'given' information, determining the 'find', and using the balanced equation to establish the relationship between reactants and products, will make the process less daunting. Another tip is to practice with everyday examples, like cooking recipes, to relate stoichiometric principles to real-life scenarios and reinforce understanding.

Double-Replacement Reaction

Double-replacement reactions are a type of chemical reaction where parts of two compounds switch places forming two new compounds. Such reactions typically occur in aqueous solutions and involve the exchange of ions between the reactants. In the reaction between sulfuric acid and potassium hydroxide, the hydrogen ions (H⁺) in sulfuric acid switch places with the potassium ions (K⁺) in potassium hydroxide, producing potassium sulfate and water. This switch can be thought of like two couples exchanging partners in a dance.

For better understanding, students should recognize that double-replacement reactions generally follow a pattern where the cations and anions in two compounds switch places. Using a hands-on approach, such as using colored blocks or beads to represent different ions, can help students visualize and understand the ion exchange that occurs. Moreover, knowing the general form of the reaction, AX + BY -> AY + BX, where A and B are cations and X and Y are anions, can be a useful mnemonic.